1. Field of the Invention
The invention relates to a method for casting prefabricated products, in particular, of steel materials such as slabs, blooms, beam blanks etc., in a strand casting device comprising a strand guide arranged downstream of the casting mold and optionally divided into segments, wherein the strand guide is comprised of pairs of oppositely arranged rolls for supporting and conveying the solidifying cast strand, wherein at least one of these rolls is forced by means of a drive, the drive roll, with a defined adjusting force, in interaction with idle rolls, against the cast strand for transmitting the guiding forces as well as strand conveying forces. The invention also relates to a device for performing the method.
2. Description of the Related Art
In continuous strand casting devices, preferably for steel materials and for manufacturing slabs, inter alia thin slabs, blooms, or beam blanks, the produced strands are supported and guided from the casting mold to at least the point of complete solidification by means of a roll guide. Individual rolls within this strand guide or downstream thereof are driven for overcoming the removal resistance to which the strand is subjected on its way through the roll guide device. The power of these drives is usually dimensioned such that, on the one hand, for any conceivable operational situation a safe conveying of the strand out of the device is ensured and, on the other hand, the manufacturing costs are as low as possible, and, moreover, the drives are not unnecessarily oversized.
The rolls of a strand guiding stretch of the strand casting device are subjected to continuous wear. Often it is observed that the driven rolls wear more strongly than the idle rolls. As a result of this, the roll diameter will change gradually. This causes diameter differences between the idle rolls and the driven rolls which are subjected to greater wear; however, the differences are not easily recognizable.
For ensuring a frictional connection between the driven rolls and a strand, it is known to use hydraulically or mechanically adjustable lifting traversing slides. Often, in particular, for avoiding quality reduction of the cast end product, they are loaded with a force which is below the force that can be produced by the strand itself as a result of its hydraulic height (ferrostatic pressure). This is designed to prevent strand deformations. However, in many cases the driven rolls are rigidly connected and integrated in a support frame.
With the wear of the driven rolls being faster in comparison to the idle rolls, after a longer or shorter operating time, a gradual loss of transmittable removal forces acting on the strand will occur which is generally not recognizable. Such a loss of transmittable removal force is limited by adjusting drive rolls by means of the applied adjusting force; however, in the case of rigidly arranged drive rolls the loss can cancel the entire transmittable tangential force. When such a creeping change of the removal force coincides with a critical casting situation relative to the removal resistance, the strand may get stuck in the casting device and this can cause a greater disruption.
In order to transmit the physically greatest possible traction force of the driven rolls onto the strand for the purpose of avoiding such disruptions, it has been suggested in this connection, for example, to expand the drive control by means of a traction control, processing simultaneously the drive torque and the rotational speed, in a so-called anti-lock system. Such a development known from the prior art is able to completely use the physical limit of the traction force defined momentarily by the material pair and the coefficient of friction. However, it cannot prevent falling below the adhesion limit, for example, resulting from a diameter loss that is too great and thus traction loss, even if, for example, by empirically determined operating experiments, the adhesion limit were known.
European patent document EP 0 908 256 A1 describes a method and a device for producing slabs in a strand casting device which has a strand guide arranged downstream of the casting mold and divided into two-part segments, wherein the casting strand is conveyed from the vertical casting direction into the horizontal rolling direction and is supported during the deflection and transport. The adjusting forces required for transmitting the conveying forces for the drive rolls are generally provided by hydraulic cylinders. In this connection, it is proposed to replace the hydraulic clamping cylinders, which clamp the two segment frame parts against one another via spacers, by segment adjusting cylinders so that the adjusting forces for the drive rolls are displaced toward the segment intake or segment exit and the forces for strand conveying are supplied also by the segment adjusting cylinders.
For strand guiding frames of a segment configuration, as described in an arc device disclosed, for example, in German patent document DE 1963 146 C1, which guides the cast strand from the vertical casting direction into the horizontal rolling direction, the upper and lower frames of the segments are clamped against one another by four hydraulic cylinders that connect the frames and are arranged on the corners external to the casting strand. The adjustment of different strand thicknesses is realized by spacers against which the frame parts are pressed. A change of the roll spacing during the casting process is not possible in this configuration.
In the German patent document DE 43 06 853 C2 it is suggested to arrange between the spacers and the respective lateral frame part a hydraulic plunger cylinder and to dimension its annular piston such that in the pressure-relieved state it secures the segment parts at the spacing of the rolls which corresponds to the desired strand thickness. By this measure, an adjustment of the guide roll to different strand thicknesses is possible, but this configuration requires that at least one of the rolls, the drive roll, is pressed by its own hydraulic adjusting cylinders (at least two per drive roll) with the required adjusting force for the transmission of the strand conveying forces against the cast strand.
It is an object of the present invention to improve the guiding of the cast strand, preferably from the vertical casting direction into the horizontal rolling direction, such that a gradual wear resulting on the rolls during the course of operation and entailing a loss of transmittable removal force can be anticipated in order to prevent by a timely initiation of preventive maintenance an operational failure, for example, preventing the strand from becoming stuck in the casting device.
In accordance with the present invention, this is achieved in that all drives are controlled to have a predetermined rotational speed such that their circumferential roll speeds are identical within the limits of a preset casting speed and the correlated motoric torques (engine torque) are maintained, with respect to the loadability of the strand shell, to a level as equal as possible below a permissible torque limit value and that, for determining the permissible torque limit values, first on a single drive of the device, while all other drives are controlled accordingly to the aforementioned nominal casting speed, the drive torque, beginning at zero, is gradually increased and, in this connection, the rotational speed of the drive roll is monitored and, in the case of a superproportional rotational speed increase A, the torque limit value B is determined and the process stopped.
With an expedient configuration of the method according to the invention it is proposed that the torque limit value B with corresponding rotational speed (slip torque and slip speed), measured immediately before the rotational speed increase A, taking into consideration boundary parameters such as the steel quality, the casting speed, the casting shape, the spraying pattern etc. with respect to the measured roll is stored in a memory for later processing.
A further development of the method according to the invention is that the measurement of the rotational speed limit value A as well as the torque limit value B and their storage is performed successively for all driven rolls of the device and the measurements are performed either manually or according to a program, wherein the measurements are sequentially repeated and performed at least once after sufficiently long running-in time of the device with lowered casting speed on a solidified area of the casting strand.
Moreover, it is provided with the method according to the invention that the aforementioned measurements are carried out in each sequence, i.e., between beginning of casting and end of casting, or once for a preset time frame, respectively, the longest however being a monthly interval.
Finally, one embodiment of the method according to the invention proposes that the measured stored data are entered into a processing module which correlates the determined parameters for slip torque and slip rotational speed, including the casting boundary conditions, linearly or squarely, for example, according to the method of the least squares, and a trend curve, which is determined thereby, is compared to a curve for the adhesion limit, based on experiments or theoretical considerations, wherein the trend curve and the limit curve Y are intercepted with one another and the point of interception defines the remaining time until reaching the functional limit of the drive rolls, and wherein planned operating programs such as maintenance times etc. are taken into consideration. This increases the forecasting precision when an approach as closely as possible to the permissible limit is reached and processing is performed preferably in a processing unit of the device with automated data transmission.
As can be seen from the method according to the invention, the basis of the invention is the rotational speed control of a strand guide of the aforementioned configuration with underlying torque control (load compensation control), wherein primarily all drives are controlled to a rotational speed such that their circumferential roll speeds are identical within permissible limits, that, however, at the same time also the motor currents (motor torques) are maintained at a same level as much as possible relative to the loadability of the strand shell within the device.
With the method according to the invention the following control for the roll drives can be performed:
1. While all other roll drives are controlled according to the actually adjusted nominal casting speed and a nominal torque, the drive torque is gradually increased for an individual roll drive of the device, starting at zero. In this connection, the rotational speed of the drive roll is permanently monitored and the process is stopped as soon as a torque has been reached at which suddenly, or superproportionally, the rotational speed changes.
2. The torque limit value and the corresponding rotational speed (slip torque and slip rotational speed), measured immediately before the abrupt rotational speed change, are stored with consideration of momentary boundary conditions such as steel quality, casting width and casting thickness, spraying pattern and/or casting speed with respect to the roll currently being measured as well as with respect to the roll segment of the roll currently being measured in a long-term memory for access and processing at a later point in time.
3. The measures defined under items 1. and 2. are performed successively for all driven rolls of the casting machine. These measurements are performed manually or according to program.
4. The measures mentioned under items 1. to 3. are repeated sequentially, at least once, however, in stationary casting operation, after a sufficiently long running-in time of the casting device, at lowered casting speed, wherein it is to be ensured that the roll employed for a measurement, respectively, runs on a completely solidified area of the cast strand.
5. The measures named under items 1. to 4. are performed regularly, for example, in each sequence, between a new beginning of casting and end of casting or once within a reference time period, at the longest at a monthly spacing.
6. The measured and stored data are correlated in an evaluation module with the boundary condition parameters recorded during the preceding measurements, wherein a trend curve for rotational speed and slip is intercepted with a limit curve wherein the points of interception identify the expected time before reaching the functional limit of the drive rolls.
A device for casting pre-fabricated products, in particular, of steel materials such as slabs, blooms, beam blanks etc., for performing the method according to the invention comprises means for storing and transmitting measured data, in particular, of the driven rolls and optionally of the segments correlated therewith to a data acquisition system of the device. According to the invention, it is proposed that the data acquisition system comprises an algorithm unit for correlating the average wear of idle rolls with that of the driven rolls, wherein the algorithm unit is connected in a data-technological way with an information unit for operational data, for example, from operating experiments or theoretical calculations based on the prior art.
Further developments of the invention can be taken from the dependent claims.